1. Field of the Invention
The invention relates to a cement composition comprising
a) 10 to 35% by weight of cement, in particular portland cement PA1 b) 65 to 90% by weight of a synthetic pozzolan obtained by an at least partial reduction from combustion residues having an SiO.sub.2 /CaO ratio of &gt;1, such as e.g. waste incineration slags, and containing alkali oxides (Na.sub.2 O and K.sub.2 O) in amounts exceeding 1.5% by weight, PA1 c) an activator for reducing the positive surface potential of the synthetic pozzolan, such as e.g. anionically active or anionic tensides, in particular, sulfonates. PA1 a) 10 to 35% by weight of cement, in particular portland cement PA1 b) 65 to 90% by weight of a synthetic pozzolan obtained by an at least partial reduction from combustion residues having an SiO.sub.2 /CaO ratio of &gt;1, such as e.g. waste incineration slags, and containing alkali oxides (Na.sub.2 O and K.sub.2 O) in amounts exceeding 1.5% by weight, and PA1 c) an activator.
2. Prior Art
Cement compositions containing cement and, in particular, portland cement and additionally more or less large portions of extenders or fillers are known. Also has already been proposed to employ cement along with natural pozzolans or synthetic pozzolans as well as carbonates as extenders, the majority of such extenders or fillers being limited merely in view of the desired strength values. With some such mixtures, demixing by sedimentation may occur, this applying, in particular, to fly ash, sand or blast furnace slag. The risk of demixing by sedimentation is faced, in particular, with low surface charges, wherein pozzolans, fly ash, microsilica and blast furnace slag, as a rule, have low to clearly negative surface charges. Pozzolans having clearly negative surface charges stand out for their good miscibility with cement, in particular, in the alkaline range. The same holds for micro-silica powders exhibiting extremely negative surface charges. Investigations have demonstrated that the respectively measured negative surface charge quite definitely is a function of the pH, wherein low surface charges remaining nearly constant over the total pH range were measured for fly ash and blast furnace slag.
Mixtures to be used for the production of coarse- and/or fine-grain-flux-containing concrete comprising 70 to 35% by weight of cement and 30 to 65% by weight of latent hydraulic substances as well as 0.05 to 1% by weight of a dispersant such as, for instance, lignin sulfonate have become known from DE-A1 31 05 054. In DE-A1 31 05 054, decreasing strength values were measured with increasing amounts of synthetic pozzolans.
Synthetic pozzolans and, in particular, those obtained by an at least partial reduction from combustion residues having an SiO.sub.2 /CaO ratio of &gt;1, such as e.g. waste incineration slags, and containing alkali oxides in amounts exceeding 1.5% by weight are characterized by positive surface charges. Consequently, the admixture of such synthetic pozzolans usually has not resulted in enhanced strength properties, in particular, in terms of early strength. Due to the formation of agglomerates, mixed cement exhibits poor working properties. As expected, such cement mixtures have, therefore, strongly fallen short of the early strength values measured for portland cement or cement compositions containing other extenders or fillers.